Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treating, melting, and annealing. Such torches include an electrode which supports an electric arc that extends from the electrode to a workpiece. A plasma gas such as an oxidizing gas is typically directed to impinge on the workpiece with the gas surrounding the arc in a swirling fashion. In some types of torches, a second shielding gas is used to surround the jet of plasma gas and the arc for controlling the work operation. In other types of torches, a swirling jet of water is used to surround the jet of plasma gas and the arc and impinge on the workpiece for controlling the work operation.
One characteristic of existing plasma arc torches is that there is little or no commonality between shielding gas torches and water-injection torches. Thus, a user who desires to employ both gas-shielded and water-injected plasma arc processes must purchase two complete torch assemblies. Furthermore, a plasma arc torch manufacturer who desires to make both types of torches must manufacture and maintain inventories of two complete sets of different components, and therefore the cost complexity of the manufacturing operation are increased.
In a typical plasma arc torch, the plasma gas and the shielding gas or water are directed by a nozzle assembly having a plasma gas nozzle and a shielding gas or water injection nozzle coaxially arranged concentrically or in series. The nozzle assembly is electrically conductive and is insulated from the electrode so that an electrical potential difference can be established between the electrode and the nozzle assembly for starting the torch. To start the torch, one side of an electrical potential source, typically the cathode side, is connected to the electrode and the other side, typically the anode side, is connected to the nozzle assembly through a switch and a resistor. The anode side is also connected in parallel to the workpiece with no resistor interposed therebetween. A high voltage and high frequency are imposed across the electrode and nozzle assembly, causing an electric arc to be established across a gap therebetween adjacent the plasma gas nozzle discharge. This arc, commonly referred to as a pilot or starting arc, is at a high frequency and high voltage but a relatively low current to avoid damaging the torch. Plasma gas is caused to flow through the plasma gas nozzle to blow the pilot arc outward through the nozzle discharge until the arc attaches to the workpiece. The switch connecting the potential source to the nozzle assembly is then opened, and the torch is in the transferred arc mode for performing a work operation on the workpiece. The power supplied to the torch is increased in the transferred arc mode to create a cutting arc which is of a higher current (and typically a lower voltage) than the pilot arc.
Because of the relatively high voltages and currents used in such torches, the electrode and nozzle assembly become hot and must be cooled to prevent early failure of the torch. Accordingly, high-current plasma arc torches generally include coolant circuits for flowing a coolant around the nozzle assembly and/or the electrode. The liquid coolants used often are capable of conducting electricity to some extent. In water-injection torches, unless deionized water is used for the injection water, the injection water is also capable of conducting electricity to some extent. In addition, some shielding gases are conductive, such as argon.
One of the problems with some existing plasma arc torches is current leakage between the electrode potential and the nozzle potential caused by injection water, shielding gases, and/or coolant flowing between adjoining surfaces of various parts of the torches and making its way from a part at electrode potential to a part at nozzle potential. When this happens, a larger voltage potential must be imposed across the electrode and nozzle assembly in order to establish the starting arc. If the current leakage is severe enough, starting the torch can be difficult or nearly impossible with reasonably manageable levels of voltage.
Another problem with some existing torches is that the shield gas or injection water typically flows through a component of the torch which is at electrode potential and then comes into contact with a component of the torch at nozzle potential over a path of relatively short length. Depending on the shielding gas or the type of injection water used, it is possible for current to leak via this path through the shielding gas or injection water. Thus, even if adequate precautions are taken to seal connections between parts to prevent wetting of adjoining component surfaces, there is still a potential leakage path which can make starting the torch difficult.
A further disadvantage of some existing torches is that the electrical conductor wire which is connected to the nozzle assembly is routed internally through the torch and is secured by a set screw in a hole in a contact ring with which the nozzle assembly makes contact when the torch is assembled. The contact ring must frequently be removed and replaced to enable replacement of certain parts that wear out. When replacing the contact ring, it can be difficult to engage the end of the conductor wire in the hole in the contact ring, especially if the end of the wire is frayed or bent. Moreover, the wire can become pushed back into the torch if there is interference between the contact ring hole and the wire.
In summary, existing plasma arc torches are subject to several disadvantages, namely, lack of commonality between gas-shielded and water-injection torches, current leakage through various leakage paths, and difficulty making an electrical connection between the nozzle and the conductor leading to the power supply when assembling the torch.